Barca's official website reported on Tuesday that he trained alone due to his hip problem:

After the draw against Elche, Barça were back in training this morning at the Ciutat Esportiva Joan Gamper as they began preparations for Saturday's title showdown with Atletico Madrid.

Of the team's injured players, Gerard Piqué began training on the pitch, though still apart from the rest of the group whilst Neymar and Jordi Alba did specific recovery work.

Updates from Monday, April 14

AS (via Rac1) reports Gerard Pique will not play in the Copa del Rey final on Wednesday:

Original Text

Gerard Pique was forced to exit Barcelona's Champions League clash with Atletico Madrid on Tuesday in the 11th minute after picking up an injury.

After colliding with Atletico forward Diego Costa, the Barca centre-back told manager Gerardo Martino that he needed to come out of the game, per the club's Twitter account:

The Associated Press (via Fox Sports) announce Pique has a hairline fracture in his hip:

Barcelona says defender Gerard Pique will be sidelined for four weeks after sustaining a hairline fracture of his right hip in Tuesday's 1-1 Champions League quarterfinal draw against Atletico Madrid

You can see the incident below:

Pique was replaced with Marc Bartra:

Kieran Canning of the Agence France Press thinks the injury could have a massive impact on Barcelona:

Pique has undoubtedly been a key part of the Barca defence this season. The reigning Spanish league champions have had their fair share of problems at the back, so to lose a player of Pique's ability can be tough to overcome.

Not only could this adversely affect Barcelona in the Champions League, but they're also a point back of Atletico in the league table. Getting back on top may be impossible if Pique is forced to miss an extended period of time.